Conventionally, JP-A-2008-35964 teaches a method for determining sleepiness of a driver of a vehicle. In the method, frequency analysis such as Fourier transformation is executed to time series data of a period of heart rate, so that heart rate variability (HRV) rate is retrieved. Based on the heart rate variability, the sleepiness of the driver is determined.
When the state of the driver of the vehicle is determined, it is preferable to determine not only a sleepiness state (or asleep state) of the driver but also an absentminded state, in which the driver is in a daze before the driver gets sleepy. However, in the prior art, there is no suggestion about determination of the absentminded state.
Further, the frequency analysis with using the Fourier transformation is much affected by a noise such as a body motion of the user and a vibration of the vehicle. Specifically, when data with the noise is Fourier transformed, spectrum power data obtained by the Fourier transformation is largely varied with a magnitude of the noise. Thus, the accuracy of the spectrum power data is low. In the above method, the sleepiness is determined with utilizing the frequency analysis, which is susceptibility to noise. In some cases, the accuracy of determination of the state of the user is low.